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<h1 data-toc-skip>High Performance OS - caladan</h1>

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  <p>Caladan: Mitigating Interference at Microsecond Timescales</p>
<h3 id="1-场景"><span class="me-2">1 场景</span><a href="#1-场景" class="anchor text-muted"><i class="fas fa-hashtag"></i></a></h3>
<p>交互式、数据密集型Web服务 —— 可变负载。在真实场景，资源使用以及干扰频繁变化的情况下，隔离分区式的解决方案存在低CPU利用率（负载低）和长尾时延（负载高）的问题。即便有些方案增加了CPU资源流动的设计，但是从负载变化到资源流动收敛，需要毫秒级甚至是秒级的时间，无法应对短时的负载突变。</p>

<h3 id="2-技术概述"><span class="me-2">2 技术概述</span><a href="#2-技术概述" class="anchor text-muted"><i class="fas fa-hashtag"></i></a></h3>

<p>基于调度时延、内存带宽、LLC未命中率等指标，实现上层任务的调度，以及CPU的流动，保持高CPU利用率和严格的性能隔离（吞吐量和尾时延）。
从负载突变到CPU流动收敛可以达到微妙级别。</p>

<p>1、微妙级的性能指标监控（调度时延、内存带宽、LLC未命中率），实时反馈当前系统负载，迅速应对负载突变的情况。
调度时延的测量基于runtime来实现。所有的业务进程都运行在caladan定制的runtime上，业务进程需要划分线程类别（高、低优先级，用于配置调度策略）
，并完全由caladan调度器管理。调度时延由线程在调度队列中等待的时间来确定。内存带宽的测量依赖于intel的性能监控器
(https://intel-pcm-api-documentation.github.io/classServerPCICFGUncore.html)，直接调用intel的API（getMCCounter），然后加以分析判断。</p>

<p>2、微妙级的响应和CPU资源流动，迅速收敛应对负载突变。
caladan的调度器每10us检查一次负载情况，响应速度很快。同时caladan舍弃了传统的资源隔离分区的做法，CPU资源流动不需要通过硬件或者linux CPU热插拔
机制来完成，而是通过业务线程的调度（原线程迁移，目标线程运行，这个过程实际上就完成了一次CPU资源流动），大大降低了开销。调度由用户态调度器和内核模块ksched共同完成，舍弃sched_setaffinity这种长路径实现。用户态指定调度策略，通过ioctl下发调度命令，并通过mmap共享内存减少用户态内核态同步开销，ksched则在接收到用户态的调度命令后，使用IPI中断在目标核上快速完成调度切换。</p>

<h3 id="3-方案缺陷"><span class="me-2">3 方案缺陷</span><a href="#3-方案缺陷" class="anchor text-muted"><i class="fas fa-hashtag"></i></a></h3>
<p>1、兼容性问题。业务进程需要运行在定制的runtime上，调度时延的测量以及线程调度都依赖于runtime对业务进程的拦截和封装。并且runtime不完全兼容linux，
包含自定义的线程、互斥锁、条件变量和同步I/O机制等。业务进程需要将内部的线程信息传递给runtime，这可能需要对现有代码进行修改。
2、暂时没有做numa节点之间干扰的优化。</p>

<h3 id="4-延伸思考"><span class="me-2">4 延伸思考</span><a href="#4-延伸思考" class="anchor text-muted"><i class="fas fa-hashtag"></i></a></h3>
<p>caladan衡量负载情况的指标直接有效，值得参考。但是对性能指标的高效监控是定制化的（runtime+intel API），定制化的垂直优化很容易带来性能提升，
但往往有兼容性问题，需要根据实际场景来取舍。在去除runtime这一层的情况下，寻求达到负载的快速感知和CPU资源快速流动的方法也是一个可以思考的方向。</p>

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  Sep 11, 2020
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              <h4 class="pt-0 my-2" data-toc-skip>The Kernel Concurrency Sanitizer (KCSAN)</h4>
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                  1 kcasan

2 使用

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                  1 kprobe

2 debugfs kprobe trace
kprobe trace 使用debugfs的trace功能
 p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]  : Set a probe
 r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]  : Set a r...
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              <h4 class="pt-0 my-2" data-toc-skip>Kernel userfaultfd</h4>
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                  Userfault只支持匿名页，hugetlb、共享内存；

一、软件流程

1 初始化
调用__NR_userfaultfd syscall初始化
调用syscall初始化建立匿名inode文件，并初始化file-&amp;gt;private_data,并返回用户态文件fd。


  
    用户态：

    uffd = syscall(__NR_userfaultfd, O_CLOEXE...
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